ProjectSummary Aging is a complex process that involves numerous physiological and morphological changes.Althoughaginginmulticellularorganismsisinevitablymorecomplex,thebasic mechanismsofcellularagingappeartobeconservedacrossorganismsrangingfromthe single-celled yeast to mammals. Cell asymmetry and polarity are critical for cell proliferation and development, and the loss of cell polarity and asymmetry has been implicated in cellular aging. However, the causes of cellular aging remain poorly understood.Cdc42,anevolutionallyconservedRhoGTPase,isoneofthekeyregulators of cell polarity in diverse species including yeast and humans. While the mechanism underlying Cdc42 polarization has been extensively investigated for its roles in the establishmentofcellpolarity,thefunctionalsignificanceoftheCdc42signalinginaging has not been addressed. Here, we will use our expertise in the small GTPase field to understandthespatialandtemporalregulationofCdc42signalingduringagingprocess. Inthisproposal,wewilltestanunexploredconceptthattheintrinsicgeneticprogramof cellpolarityandmorphogenesisislinkedtocontrolofcellularlifespanusingthetractable budding yeast as a model. Specifically, we will explore how changes of positive and negative regulation of Cdc42 during repeated cell divisions limit replicative lifespan by combiningmethodsingeneticsandlive-cellimaginginamicrofluidicdevice.Wewillalso use mathematical modeling to predict underlying mechanisms leading to loss of Cdc42 polarization and thus limiting the cell division. The outcomes of this work will lay the foundation for identifying a mechanism underlying cellular aging that will be also applicable to other eukaryotes. Knowledge gained from this study will ultimately be translatable to identifying candidate genes and processes in humans that are similarly affectedbyaging.